This invention relates to an extrusion apparatus, more particularly a screw type extrusion apparatus which is useful in the blending of low viscosity additives into high viscosity materials such as thermoplastic melts.
Extrusion devices are well known in the art for the processing of a wide range of materials. Of particular interest to this invention are extruders of the screw type, in which one or more screw worms rotate within a closely fitting peripheral barrel. Rotation of the screw applies shear forces to an extrusion stock and propels the stock continuously through one or more regions wherein it is compressed and compacted, heated, softened, metered, and/or worked. In one very common service, the screw extruder is applied to produce a uniform, continuous stream of a molten thermoplastic extrudate suitable for injection into a mold or extrusion through a die, or otherwise useful for materials forming or further processing.
In practice, screw extruders are also often called upon to serve a mixing function. It may be desired, for instance, to mix or blend one or more additives into the extrusion stock as it is passed through the extruder. In some cases, an extruder is dedicated to a mixing (or, equivalently, a compounding or blending) operation. In other cases, additives are introduced as a matter of convenience into the extruder which has a principal function other than mixing. This is often true of colorants, stabilizers, and the like which could be mixed with the extrusion stock in a separate mixing step, although at increased equipment and operating expense. In still other cases, the additive may be one which provides a critical contribution to the particular application of extrusion and is necessarily added directly to the extruder. In this regard, mention may be made of coolants blended into the extrusion stock in order to moderate the buildup of heat which is generated by friction in a viscous extrusion stock and which may lead to thermal degradation of the stock. Mention may also be made of devolatizing agents which are commonly applied to aid in the separation of volatile substances, e.g., residual solvents, which represent a purity or safety problem and/or which result in porosity, bubbles, blisters, and similar defects in a formed extrudate product.
Even though screw extruders are commonly applied for mixing, they are recognized in the art as being rather poorly suited to this function. This is particularly true of single-screw extruders in which a motion of the material through the extruder is substantially in a uniform (helical) direction, and there is little opportunity for volume blending. Multiple-screw extruders typically offer improved mixing, relative to single-screw extruders, but are still often less effective than would be desired. In essentially all applications of screw extruders, the high viscosity of the extrusion stock inhibits turbulent flow or diffusion which would contribute to mixing within the extrusion stock.
The principal object of this invention is a screw extrusion apparatus having enhanced capabilities for the mixing of the primary extrusion stock with one or more lower viscosity additives. It is further the object of this invention to provide improved mixing efficiency by a means which does not significantly increase the expense of extruder equipment or operation, or have adverse effect upon product quality. In this respect, methods conventionally applied to increase mixing efficiency of such an extrusion involve increased working of the extrusion stock, for instance through the use of a lengthened screw and barrel, and/or application of greater force to the screw rotation. Such approaches increase not only equipment costs and power requirements, but also residence time and temperature, and hence degradation, of the extrusion stock.
A critical feature of the apparatus of the invention is an injector for introduction of the additive through the barrel of the extruder and into the path of the primary extrusion stock. The injector extends through the barrel and inwardly into and substantially through the annular space provided for flow of the extrusion stock, to a point essentially at the surface of the screw shaft (or root). In prior art practice, it has been most common for screw extruders to be provided with provisions for the introduction of additives by means of a port or injector terminating at the barrel wall. One prior art disclosure, that of U.S. Pat. No. 4,155,690, describes a screw extruder for the production of crosslinked polyethylene, in which the cross-linking agent (dicumyl peroxide) is added to the polyethylene melt in the extruder through an injector which extends half way between the barrel wall and the screw root. This patent suggests that such a position of injection avoids pulsed back pressure on the dicumyl peroxide supply system.